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Hey everyone so I am really confused here. I have this 2003 Impala LS with a 3.8 v6. The car will sometimes not start unless I press the gas pedal. This usually occurs when its hot but not always. It will crank but sometimes will not tun over unless I just so slightly tap the gas, and I mean put the slightest pressure on it. fuel pump is new, fuel filter is new, fuel pressure regulator is new, MAF is new, IAC is new, Crank sensor is new, fuel pressure is great. the car has 110% power no hesitation and will fire right up as soon as I hit the gas. The only noticeable difference is a slight decrease in occurrence since I added some injector cleaner to the fuel tank. but the problem still persists. No vacuum leaks either, I have smoke tested every hose under the hood. Air filter is also new. Every sensor is sending signals, no check engine light. The car also has spark whenever this occurs. All I can think is that the engine is not getting any air but I just don’t understand how that is possible. Or clogged injectors, but I also don’t see how that could be possible either. The car sat for a year but I have ran at least 3 full tanks of gas through it so far. I just don’t understand how moving the throttle plate just the slightest bit will get it to immediately fire. once it is running it also idles perfect right at 800-900 rpms
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Man, this is puzzling…It sounds like you’ve covered all your bases. I’m almost embarrassed to ask, because I can tell you are a thorough mechanic. Did you clean the throttle plate? It sounds like there might be some kind of restriction in the fuel line. Are the lines in good shape, crimped or bent? Since the fuel rail is not horrible to get to on that make, maybe you could take it off, remove the injectors and blow some compressed air through it…maybe the fuel lines too. Of course you said fuel pressure is good… Honestly, grasping at straws…sure would be interested to know what you find.
I’m glad you mentioned the throttle plate cleaning as it is the only thing i have not done yet. I just hadn’t considered that since they usually only need cleaned if they are sticking, but i think its definitely an air supply issue. I am thinking the throttle body might just be caked up since when I changed the IAC valve the old one was really gunked up. I’m probably going to take the whole throttle body off and clean it, I also was considering just taking the air filter out and see if it will start better which would verify n air supply problem. Note that I am not throwing parts at it all of the things I replaced were bad for the most part but I thought the problem would fix itself once I replaced them, go figure. I will let you know what I find, Thanks -Darren
Check your TPS to verify its not having a problem.
http://easyautodiagnostics.com/gm/3.8L/how-to-test-the-throttle-position-sensor-1
Yeah you were right, I hooked my scope up and graphed the sensor and it drops off and spikes at idle, and when i move the throttle plate and tap on it it spikes and drops as well. I didn’t think that the tps was bad being that it ran good and shifted fine, I assumed it was good and look where that got me. Ill verify that this fixed it once I get the sensor swapped out. Thanks again -Darren and have a great weekend everyone!
Well bad news, I changed the TPS and nothing has changed. I find this so puzzling. The throttle body is actually clean as a whistle, the egr pipes are clear, I don’t get it. It starts perfect every time on a cold start, but as soon as it runs for a little while, it doesn’t even have to get hot and then 50% of the time it starts fine and the other 50% of the time it wont unless I feather the gas.
It sounds mixture related with touching the throttle.which in this case
is air. When you go to start the car does the engine light illuminate and
go out? The coolant sensor would be next in line. read the ohms when cold
and then read ohms when it won’t start.yeah tje engine light functions properly, However this morning the problem got much worse to the point of the car not running good at all. It pops and misfires and all kinds of stuff. and there was a loud hissing sound coming from the engine, which turned out to be the upper intake gasket. It has a huge hole in it next to the throttle body, so it looks like ill be changing that now and then we will see how she runs.
Ok this is really something. I replaced the upper intake gasket, and throttle body gasket. so now the whole intake is clean and not leaking. All of the sensors on the throttle body are new, and the coolant temp sensor is also new. The car runs great, but it still has the starting issue. It will start when its cold no problem. but as soon as you drive it five minutes or five hours and you shut the car off and try to start it it just simply wont turn over. and its so weird cause you can tell it wants to, it just barely doesn’t stay running. even sometimes it will stay running but it will catch itself at 400 rpms and then it will increase the rpms. I notice that the fuel trim numbers are kind of weird like in the -10.5’s i don’t think they should be near that, should they? what would be making those fuel trim numbers so far into the negatives?especially if the MAF, Temp sensor, IAT, and TPS are all functioning properly. its just so strange. I was kind of thinking ICM since they are known failures on these cars but it always has spark when it wont start.
Are stft and ltft both that negative? You are running rich and subtracting
fuel. Two things MAF and primary o2. What is the primary o2 doing? Are
you going into closed loop? Is the primary o2 switching rapidly?o2 b1s1: 0.7 V
o2 b1s2: voltage is all over the place, erratic
stft: 0 at idle, goes up as i give it gas
ltft: -16 at idle and -22 when i give it gas.however, I made the discovery that when I remove the brake booster hose the car starts perfectly. so does that mean that its starved for air? and I forgot to mention that I am only getting 200 miles to a tank of gas so something is not right.
[quote=”Durango8766″ post=114189]o2 b1s1: 0.7 V
o2 b1s2: voltage is all over the place, erratic
stft: 0 at idle, goes up as i give it gas
ltft: -16 at idle and -22 when i give it gas.however, I made the discovery that when I remove the brake booster hose the car starts perfectly. so does that mean that its starved for air? and I forgot to mention that I am only getting 200 miles to a tank of gas so something is not right.[/quote]
If it was an air problem it would be there all the time. bank one
o2 should be switching rapidly. Have you checked fuel pressure running?
Also do a FP leak down test to see if fuel pressure drops off. Either leaky
injectors or FP regulator. Does bank one o2 stay at .7?fuel pressure when running is 52 psi at idle, my fuel pump and fuel pressure regulator are both new. after i turn the car off the fuel pressure goes down to 35 after 2 or 3 minutes. bank 1 sensor 2 O2 sensor stays at .7 all the time. bank 1 sensor 1 O2 rapidly changes.
[quote=”Durango8766″ post=114189]
however, I made the discovery that when I remove the brake booster hose the car starts perfectly. so does that mean that its starved for air? and I forgot to mention that I am only getting 200 miles to a tank of gas so something is not right.[/quote]
Maybe the check valve on the brake booster is causing a vacuum leak, which would explain the poor gas mileage.
So I think I fixed the car, or at least made it start correctly. After taking the line off of the booster I got thinking. Well I decided to remove the pcv and try running the car. What do you know it fires right up. So i try sucking on the pcv(yeah I know what your thinking) and it kind of stuck at first or at least hard to move air through it. so after blowing and sucking on this oily valve it feed up. and now the car starts fine 95% of the time. So Ill be picking up a new one tomorrow. I think the spring inside is collapsed. I don’t know if this explains the odd fuel trim numbers and the horrible mileage but as bad as this sounds I am selling the car so it wont be my problem anymore. I hate being brand bias because all cars have their issues, but this is by far the worst vehicle I have ever owned, and I wont be buying any GM product of this vintage any time soon. Thanks everyone for your help and suggestions, it is greatly appreciated.
Glad its working out. I would be curious about the fuel trims. 🙂 I’m
also curious with the fuel pressure drop. Is it an injector, FP or FPR
I know 2 of the three are new. My theory is the injectors leaking and
creating a hot soak and the extra air is needed to clear it. 🙂 The bank
1 o2 being stuck rich. Keep us posted on your progress.
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